The invention relates in general to automatic and semiautomatic gear and gear door extension/retraction systems, and more particularly to systems having pilot operated, alternate (backup) gear extension.
Two separate gear and gear door control systems are provided on modern commercial transport aircraft. A first of these systems is called the alternate gear extension system which provides a pilot backup to the primary gear and gear door extension control. Should the primary gear extension system fail, the alternate extension system can be used by a pilot to free the gear door and allow the gear to fall by gravity (or, in some cases, by an auxiliary gear release actuator) into an extended, prelanding condition. Early alternate extension systems included cables and associated mechanical levers and catches controlled from the cockpit. More recent innovations have replaced the traditional cable actuating system with hydraulics and electro-hydraulics in which the hydraulic components are similar to a car braking system. Since the alternate gear extension is a safety backup, which is not normally needed, the hydraulic components of the system are not frequently exercised and, hence, there is an increased chance of some trouble developing in the hydraulic system, for example, a dry seal, which could cause the system to malfunction. This is analogous to the increased probability of failure of the hydraulics of a vehicle's brakes after a prolonged period of non-use.
As a previously separate subsystem of the aircraft, many modern transports are equipped with a gear door release that can be actuated by the ground-crew working beneath the belly of the aircraft to cause the gear doors to open for access to the wheel wells. It will be noted that the gear and gear door (or doors) operate in a coordinated sequence on approach to landing in which the gear door is first opened, followed by the extension of the landing gear, and completed by the reclosing of the gear door. Thus, when the aircraft is on the ground with gears extended, the gear doors are closed and must be opened in order to facilitate maintenance by the ground-crew. As mentioned, many aircraft are equipped with a subsystem for causing the gear doors to open in response to a switch or latch located near the door on the belly of the aircraft and accessible by the ground-crew. This same subsystem includes means for operating the door closing actuator to close the door when the maintenance task has been completed. In the prior systems for ground-crew release (and subsequent closure) of the gear doors, an entirely mechanical linkage has been used to connect a release/close handle on the aircraft's underside to a lever lock on the gear door actuator, and this mechanical means is accompanied by a significant weight penalty.